Get screen space coordinate for node position
There are two utility functions for regions to convert 2D spaces:

Region.view2d.region_to_view()
Region.view2d.view_to_region()


-- API Docs

You might not even need it, try 'POST_VIEW' for your draw callback:

bpy.types.SpaceNodeEditor.draw_handler_add(draw_callback_px, args, 'WINDOW', 'POST_VIEW')


This set, you can draw in the node space in your callback function, unlike 'POST_PIXEL', which will always give you the screen space.